The invention concerns a digital magnetic memory cell device for reading and/or writing operations with a first and a second magnetic layer, whose magnetizations are aligned parallel or antiparallel to one another for storing digital information, whereby at least one of the magnetic layers has a magnetic anisotropy as well as a method for the reading/writing of digital information to a digital memory cell device In addition, a digital memory device comprising a multiple number of memory cell devices and a method for reading/writing digital information to such a digital memory device is disclosed.
Currently, a multiple number of magnetic memories with freely selectable access (MRAM) have been developed.
An MRAM comprises a multiple number of magnetic memory cell devices. Each memory cell device comprises at least two magnetic layers, which are separated by an intermediate layer. The two magnetic layers may be magnetized parallel or antiparallel to one another. The two aforementioned states represent each time one bit of information, i.e., the logical zero (xe2x80x9c0xe2x80x9d) or one (xe2x80x9c1xe2x80x9d) state. If the relative orientation of the magnetization of the two layers is changed from parallel to antiparallel or vice versa, then the magnetoresistance typically changes by a few percent. This change in the resistance may be used for the readout from the memory cell of stored digital information. The change of cell resistance may be recognized by a voltage or current change, depending on wiring. For example, in the case of a voltage increase, the cell with a logical zero (xe2x80x9c0xe2x80x9d) can be validated and in the case of a voltage decrease, the cell with a logical one (xe2x80x9c1xe2x80x9d) can be validated.
Magnetic memory cells, which use the magnetoresistance effect for storage of digital information, have been made known from a plurality of publications.
Refer to the following in this regard:
EP 0 614,192
U.S. Pat. No. 5,343,422
EP 0 685,849
EP 0 759,619
U.S. Pat. No. 5,448,515
U.S. Pat. No. 5,276,639
U.S. Pat. No. 5,650,958
Particularly large increases in resistance in the range of several percent have been observed when the magnetization alignment changed from parallel to antiparallel and vice versa in cell structures with a giant magnetoresistance effect (GMR) or the tunnel magnetoresistance effect. Such cell structures are made known, for example, from
M. N. Balbich, J. M. Broto, A. Fert, F. Nguyen Van Dau, F. Petroff, P. Eitenne, G. Creuzet, A. Freiderich and J. Chazelas xe2x80x9cGiant Magnetoresistance of (001)Fe/(001)Cr Magnetic Superlatticesxe2x80x9d, Physical Review Letters, Vol. 61, No. 21, p. 2472 ff. and
Teruya Shinjo, Hidefumi Yamamoto xe2x80x9cLarge Magnetoresistance of Field-Induced Giant Ferrimagnetic Multilayersxe2x80x9d, Journal of the Physical Society of Japan, Vol. 59, No. 9, pp. 3061-3064;
WO 95/10112
WO 96/25740
EP 0 759,619 and
DE 197 17,123
An important advantage of magnetic memory cells, as described above, can be seen from the fact that the information is stored persistently in this type and manner, for example, in contrast to conventional semiconductor memories, and consequently after turning off the device in which the memory cells are used, and then again turning it on, the stored information is immediately available. In addition, storage media that are very resistant to radiation or xe2x80x9cradiation-fastxe2x80x9d are obtained.
A digital memory device for reading and/or writing operations has been made known from DE 195 34,856, which has a first magnetic layer and a second magnetic layer as well as a separating layer lying in between for conducting the read and/or write currents, whereby a directional change of the magnetization in one or both layers is effected, lasting over the time interval of the flowing current. The disclosure content of DE 195 34,856 with respect to a digital memory device according to the prior art is taken up to the full extent in the disclosure content of the present application.
A disadvantage of the digital memory device according to DE 195 34,856 is the fact that the addressing of the memory cell or individual memory cells of a memory cell matrix in an MRAM arrangement is conducted only by the position of the memory cell. This process is in no way optimal in regard to the time durations, particularly short or ultra-short time durations; in the case of several sequentially connected memory cells, the problem arises, in particular, that the coercive fields of the individual MRAM memory cells have a certain scatter as a result of irregularities, for example pinning centers.
As a result of this, it happens that not only is the desired memory cell addressed at the point of intersection but, rather, adjacent cells are also addressed, especially in the case of combining a plurality of the memory cell devices, which are known from DE 195 34,856, to give a memory device, e.g. in matrix form.
A magnetic memory device has become known from U.S. Pat. No. 5,448,515 that comprises a plurality of memory cells by means of which information can be a written into and read from the addressed memory cells as a result of remagnetization with the help of current pulses.
A disadvantage of this memory device was that writing in took place via currents or current pulses of unspecified time duration and the writing and reading conductors constantly enclosed an angle of 90xc2x0.
The memory device that is known from U.S. Pat. No. 5,276,639 is a superconducting magnetic memory device. In the case of this magnetic memory device, likewise, current pulses with a long duration of more than 15 ns are transformed for the purpose of writing in and reading out.
A digital magnetic memory cell device has become known from
EVERITT B A ET AL.: xe2x80x9cSIZE DEPENDENCE OF SWITCHING THRESHOLDS FOR PSEUDO SPIN VALVE MRAM CELLSxe2x80x9d, JOURNAL OF APPLIED PHYSICS, Vol. 81, No. 8, PART 02A, Apr. 15,1997 (1997-04-15), pages 4020-4022, XP000702728 ISSN: 0021-8979
whereby this device has a first and a second magnetic layer, an intermediate layer, two intersecting printed conductors as well as means for reversing the magnetization. The memory cell device that has become known from Everitt B. A. et al. (loc. cit.) operates via bit switching times of the order of magnitude of 1 ns. However, no data are given regarding the length of the current pulses.
The object of the invention is to avoid the disadvantages of the prior art that are described above and, in particular, to indicate a magnetic memory cell device in the case of which the reversing process both with respect to time as well as also with respect to the selection of the respective cells, i.e., very short switching times may also be realized. In particular, a secure and rapid switching of the addressed cells will be obtained with the combination of a multiple number of memory cells or memory cell devices into one digital memory unit, for example in matrix form or in the form of an array, even if the material properties fluctuate from cell to cell within the tolerance framework.
Another aspect of the invention is the indication of a method for optimized reversing of such a memory cell device. In addition, the invention aims at minimizing the power consumption necessary for reversing and thus minimizes the heat losses discharged from a module.
The object is resolved according to the invention by the fact that in a digital magnetic memory cell device according to claim 1, the reversal means comprise devices for producing currents and/or current pulses on a first and a second printed conductor of at least two intersecting printed conductors, wherein the printed conductors intersect at a predetermined angle, so that a complete and reliable reversal of the magnetization from a parallel to an antiparallel alignment is achieved with current pulses of a time duration of  less than 10 ns in the intersection region in the memory cell device.
If clock times of 10 ns corresponding to 100 MHz and faster are aimed at for MRAM memories, then the length of a field pulse for writing in digital information, for example, amounts to a maximum of 5 ns. Even with such short pulse durations, a stable switching must also be obtained in the case of writing processes directly following one another. The method of the invention makes available a rapid switching of MRAM memory cells, which satisfies the conditions:
(i) high stability of the switching process against fluctuations of material and pulse parameters,
(ii) high insensitivity to undesired switching of adjacent cells and of cells under the address lines,
(iii) high stability of the switching process even in the case of directly sequential switching processes of an individual cell, and
(iv) rapid switching times.
If a magnetic field is applied to a memory cell, then the direction of the magnetization shows precession over the direction of the field acting in the layer.
It is to the merit of the inventor to have made known the following for solving the object of the invention:
If the duration of the magnetic field pulse applied for switching is too long, i.e., typically longer than 5 ns, then very many precession reversals occur during the application of the switching pulse, and the switching is determined by dissipative mechanisms, described by an attenuation constant xcex1. The Zeeman energy introduced by applying the external field is energetically dissipated by an attenuation mechanism.
If the pulse length is short, typically shorter than 5 ns, the remagnetization behavior is essentially more intensely determined by the precession behavior of the magnetization, and with a further decreasing pulse length, this tendency is even stronger. The precession is barely attenuated up to the end of the field pulse, and the direction of magnetization as well as the content of the information written in depends to a great extent in which direction the magnetization lies at the end of the applied field pulse, so that the memory cell devices known from the prior art and read/write operations do not have stable switching behavior, particularly in the case of fluctuating material and pulse parameters.
The switching process in a magnetic memory cell can be described by the Gilbert form of the Landau-Lifschitz equation:                                           ∂                          M              ⇀                                            ∂            t                          =                                            -              γ                        ⁢                          xe2x80x83                        ⁢                          M              ⇀                        xc3x97                                          H                ⇀                            eff                                +                                    α                              M                s                                      ⁢                          M              ⇀                        xc3x97                                          ∂                                  M                  ⇀                                                            ∂                t                                                                        (        1        )            
Reference is made in this regard, for example, to T. L. Gilbert, Phys. Rev. 100, 1243 (1955), whereby the disclosure content of thus article is included to the full extent in the present application.
The first term on the right side of equation (1) describes the precession of the magnetization, and the second term describes the attenuation, wherein xcex1 is a phenomenological attenuation parameter.
The effective field
{overscore (H)}eff 
is defined by the sum of all fields, which act on the magnetization:                                           H            ⇀                    eff                +                              H            ⇀                    ext                +                              H            ⇀                    ani                +                              H            ⇀                    shape                                    (        2        )                                                      H            ⇀                    ani                +                              "LeftBracketingBar"                                          H                ⇀                            ani                        "RightBracketingBar"                    ⁢                                    M              x                                      M              s                                ⁢                      x            →                                              (        3        )                                                      H            ⇀                    shape                =                  -                      (                                          4                ⁢                π                ⁢                                  xe2x80x83                                ⁢                                  N                  x                                ⁢                                  M                  x                                ⁢                                  x                  ^                                            +                              4                ⁢                π                ⁢                                  xe2x80x83                                ⁢                                  N                  y                                ⁢                                  M                  y                                ⁢                                  y                  ^                                            +                              4                ⁢                                  πN                  z                                ⁢                                  M                  z                                ⁢                                  z                  ^                                                      )                                              (        4        )            
whereby
{overscore (H)}ext: externally applied pulse field
{overscore (H)}ani: is the maximum anisotropic field, whereby in the present analysis, the facilitated axis of magnetization lies in the x direction
Ni (i=x, y, z) are the diagonal elements of the demagnetization tensor in diagonal form.
Mx, My, Mz are the components of the magnetization vector in the x, y and z directions.
Solutions to equation (1) can be found, for example, by means of a Runge-Kutta algorithm, as is described in J. R. Dormand, P. J. Prince, J. Computat and Appl. Mat. 7, 67 (1981), whose disclosure content is taken up to the full extent in the present application.
By appropriate selection of the ratio of currents in the two current-bearing conductors as well as the pulse durations, a predetermined angle xcex8 of the magnetic field can be set beforehand opposite the facilitated direction of the magnetization. If the pulse duration and the angle xcex8 are set, as described later on, then stable and complete reversing of the memory cells is achieved even in the case of very short switching pulses of  less than 10 ns, and particularly of  less than 5 ns.
In a particularly advantageous configuration of the invention, the pulse durations are selected such that the complete reversal is achieved simultaneously with minimum power consumption.
In a first form of embodiment of the invention, unipolar pulses with a specific pulse duration are applied to the conductors.
In a second form of embodiment, instead of unipolar pulses on the two conductors, bipolar pulses are applied to one conductor and a static current or current pulse is applied to the other printed conductor. The bipolar pulse is tuned precisely to the reversing time of the memory cell device in the length of the first half-wave and couples resonantly to the reversal of magnetization. It is applied to the writing conductor, in which a magnetic field is produced perpendicular to the facilitated direction of magnetization in the layer to be reversed. A static current or a current pulse, whose length is longer than the half-wave time of the bipolar pulse, is applied to the other printed conductor. This produces a magnetic field perpendicular to the reversing field. In a particularly simple configuration, the printed conductors are arranged such that they are perpendicular to one another in the memory cell device, i.e., they intersect at a right angle.
Other angles xcex2xe2x89xa090xc2x0 at which the printed conductors intersect are also possible. For example, it may be advantageously provided for specific pulse durations that the printed conductors run parallel at least on a partial segment, i.e., xcex2=0xc2x0 or one of the two printed conductors runs diagonally over the memory matrix, i.e., xcex2≈45xc2x0.
The magnetic anisotropy of the layers, the basis for the formation of a direction of facilitated magnetization is preferably obtained by means of incremental processes and/or a shape anisotropy, for example, an ellipsoid edging of the layers. It is particularly preferred if the magnetization must be reversed only in one layer, whereas it is retained in the other layer, also with switching pulses. Such behavior is achieved by the fact that in a further enhanced form of embodiment of the invention, one of the two magnetic layers is magnetically harder than the other, for example, due to the fact that one of the layers is thicker than the other layer, or one of the layers has contact with an antiferromagnetic layer. In a particularly simple configuration of the invention, one of the printed conductors lies parallel to the direction of facilitated magnetization in the intersecting region.
In addition to the above-described memory cell device, the invention also makes available a method for writing digital information to such a memory cell device. The method according to the invention is characterized by the fact that currents and/or current pulses with a pulse duration of  less than 10 ns are applied onto the two printed conductors, so that in the intersecting region of the printed conductors a magnetic field at an angle xcex8 opposite to the direction of facilitated magnetization is built up by the current pulses, whereby the relative orientation of the magnetization can be changed and thus information can be written into a memory cell.
In a first form of embodiment of the invention, the current pulses can be unipolar current pulses. In a second enhanced form of embodiment, the current pulse, which is applied to the printed conductor, at which the current flux produces a magnetic field perpendicular to the direction of facilitated magnetization is a bipolar current pulse and a current or current pulse is preferably applied on the other printed conductor with a time duration, which is longer than the half-wave duration of the bipolar current pulse. The half-wave duration preferably amounts to less than 10 ns.
In an advantageous form of embodiment, it is provided to undertake the setting of angle xcex8 to the direction of facilitated magnetization by selection of the ratio of current intensities of the current pulses to one another.
The memory cell device according to the invention is part of a digital memory device with a multiple number of such memory cell devices in a preferred form of embodiment. Each of the memory cell devices has at least two intersecting printed conductors. Preferably, the memory cell devices can be applied as an array in matrix form.
In one form of embodiment of the invention, common reversing means are assigned to a multiple number of printed conductors arranged in rows or columns.
In a particularly simple form of embodiment, the magnetically harder layer of the individual memory cell devices can be extended out over several cells.
In such a digital memory device, the method according to the invention for remagnetizing a memory cell device can be applied with special advantage. In particular, such a process makes possible a precise addressing of the individual memory cells of the matrix on the basis of a selection of those memory cells by means of a time scale, in which the resulting field pulse is applied at an angle xcex8, which is exactly the case in the intersecting region of two intersecting current-bearing conductors. Remagnetization is then produced by a rotation of the magnetization from the antiparallel to the parallel alignment or orientation or vice versa. In the case of short switching times according to the invention, the angle xcex8 at which the switching magnetic field is applied, is of decisive importance, since the precession is hardly attenuated up to the end of the field pulse and the Zeeman energy introduced into the system by applying the field pulse, in order to achieve a stable switching, must again be taken from the system in the form of Zeeman energy. If this is maintained, then only the cells lying in the intersecting region of the current-conducting conductors are selected with the method of the invention, based on the pulse duration and, as the case may be, the current intensity, i.e., the reversing is produced without influencing the adjacent cells. These latter cells maintain their magnetization despite the switching pulses. Fluctuations of the pulse width or duration and of material properties clearly have a smaller influence on the stability of switching.